Critical Amplification by Gas6 of the Epo-Dependent Erythropoietic Response to Anemia: Novel Opportunities for Anemia Treatment.

Many patients with anemia fail, for unknown reasons, to respond to erythropoietin (Epo) - the key mediator of the erythropoietic response. Here, we report that loss of the growth arrest-specific gene 6 (Gas6), a polypeptide binding the receptor tyrosine kinases Tyro 3, Axl and Mer, caused a significant depletion of the reserve of erythroid progenitors in mice. As a result, the compensatory erythropoietic response to anemia induced by hemolysis or blood loss was defective in mice lacking Gas6 (Gas6−/ −). The impaired erythropoietic response was attributable to the hyporesponsiveness of Gas6−/ − erythroblasts to the survival activity of Epo. Only when recombinant Gas6 (rGas6) was administered together with Epo, did these factors synergistically rescue the mutant mice from anemia. Administration of rGas6 alone to Gas6−/ − mice also provided protection from phenylhydrazine (PHZ)-induced anemia, presumably due to the ability of Gas6 to enhance the pro-survival effects of Epo on erythroblasts. This was supported by in vivo and in vitro findings: (i) In the absence of Gas6, more erythroblasts died in the spleen of PHZ-treated mice, despite markedly elevated levels of Epo. (ii) Compared to WT, Gas6−/ − erythroblasts responded less to the pro-survival effects of Epo in vitro, suggesting that Gas6 signaling influences Epo receptor (EpoR) signaling. Absence of Gas6 did not alter tyrosine phosphorylation of the EpoR, but reduced Akt phosphorylation in erythroblasts when treated with Epo. Akt phosphorylation in Gas6−/ − erythroblasts was, however, restored to WT levels, when co-treated with Epo and rGas6. These findings indicate that Epo/EpoR-mediated erythroblast survival and proliferation is stimulated by Gas6 via the PI3K/Akt pathway. (iii) WT erythroblasts secrete Gas6 when treated with Epo. Together, erythroblasts, via a “Gas6 feedback system”, regulate, in an autocrine manner, their own responsiveness to Epo by reinforcing signaling downstream of EpoR. This mechanism is delicately balanced, since treatment of WT anemic mice with rGas6 alone reversed the acute anemia induced by hemolysis without causing polycythemia. A similar beneficial response in hematocrit restoration was observed when a transgenic mouse model with chronic anemia was treated with rGas6. In these mice, co-administration of rGas6 and Epo resulted in sustained rise in the hematocrit, to higher levels than achieved with Epo or rGas6 alone. Gas6 is therefore effective in chronic anemia, augments the effects of Epo, and may have Epo dose-sparing effects, and thus may provides novel and safe therapeutic approach to treat patients with Epo-resistant anemia.